In almost every steering wheel of a passenger car, a bus or a truck, a horn switch is located. For this purpose a movable part of the steering wheel is held on a non-movable part of the steering wheel in such a way that it can be pressed down against the force of a spring. This non-movable part is often the so-called steering wheel body and the movable part is often a part of an airbag module, for example the housing of the module (so called floating module concept) or the cover of the airbag module (so called floating cover concept). Often more than one horn switch is present, but for the sake of simplicity, reference is made to only one such horn switch in the following description.
In the simplest case, the horn switch is formed of a contact at the stationery part of the steering wheel and the mating contact on the movable part of the steering wheel. As long as no actuation force is applied to the movable part, the contacts are spaced part from another by means of the spring. If the movable part is pressed down against the force of the spring, the two contacts come in contact with one another and close an electrical circuit which leads to an actuation of the horn. One drawback of this design is that the contacts can be subjected to mechanical or electricalwear.
EP 2 326 534 describes a generic unit of the above-described type and suggests to use a load cell in form of a strain gauge in a horn switch of a steering wheel strain gage type. Load cells are resistors that change their electrical ohmic resistance in response to mechanical strain resulting from an applied force. A strain gauge is an example of a load cell. In the steering wheel described in EP 2 326 534, a strain gauge is placed between the stationery part of the steering wheel and a force transmitting part of the steering wheel, which is a movable part, in such a way that the strain gauge is stressed when the movable part is pressed down against the stationery part. The change of the electrical resistance of the strain gauge can be measured and the result of the measurement can be used for generating the horn actuation signal. In order to generate the horn actuation signal in response to the change of the electrical resistance of the strain gauge; one needs an electrical circuit, for example in form of a control and an evaluation unit. Such load cell type horn activation circuits enable horn operation without requiring displacement of the moveable component with respect to the first component of the magnitude as needed with spring and electrical contact systems. This characteristic can provide opportunities for new design features and moreover can reduce the generation of undesired buzz, squeak and rattling problems associated with movable parts.
In EP 2 326 534, the electrical resistance of the strain gauge is measured by using a Wheatstone bridge with the strain gauge or multiple strain gauges being part of the Wheatstone bridge. At the time the vehicle is started (and the horn is not pressed down) this control and evaluation unit measures the resistance of the strain gauge in its (not stressed) basic state and computes a zero value from this resistance based on a measured voltage. The resistance of the strain gauge is permanently determined by measuring an actual voltage and this actual voltage is compared to this zero value. The measured changes are interpreted and a horn actuation signal is derived from this.
Starting from this prior art it is the task of this invention to provide an enhanced device and method for generating a horn actuation signal using a load cell. Especially, it is a task to provide a more reliable horn actuation signal and to make it possible to use cost effective standard electronic elements.
This task is solved by a method and by an apparatus as described herein.